Flavin coenzyme analogs, synthetically modified at various key ring loci, will be used to analyze and factor out elements of flavin redox function, such as two electron/one electron capacity, sites of entry and exit of electrons, and nature of O2 reductive activation and splitting. Experiments will focus on mechanistic studies of the bacterial HgII reductases, cyclohexanone monooxygenases, methanogen hydrogenase, the superoxide-generating system from human leukocytes, and an S-oxygenating flavoenzyme from liver.